1. Field of the Invention
The invention relates to measuring method and an apparatus for measuring a thickness of a plate material such as plate glass, iron plate, aluminum plate, or the like.
2. Description of the Related Background Art
Hitherto, as an apparatus for measuring a thickness of plate material, there are known a simple measuring apparatus for actually measuring a thickness by using a micrometer, a caliper, or the like, a measuring apparatus for optically measuring a thickness by using a laser beam or the like, an apparatus for measuring a thickness by using an ultrasonic wave or the like, and the like.
The above conventional measuring apparatuses, however, have the following drawbacks.
First, in the apparatus for measuring by using a micrometer, a caliper, or the like, only the portion around the plate as an object to be measured can be accurately measured and, for instance, in case of a plate material having a wide area, the measurement of the plate thickness of the central portion cannot help becoming the relative measurement. In other words, in such a measurement by the micrometer or the like, a reference surface is first specified, a plate material as an object to be measured is put on the reference surface, and a distance between the upper surface of the plate and the reference surface is measured, thereby measuring a thickness of the plate material. In such a case, when the plate material as a measuring object is distorted or warped, a gap occurs between the lower surface of the plate material and the reference surface in dependence on the location of the plate material. When such a gap occurs, the measured plate thickness differs from the actual plate thickness, so that the measurement may not be accurate.
On the other hand, in the case of the optical measuring apparatus using the laser beam or the like, the apparatus itself is expensive and its handling is also complicated. In the case of the apparatus for calculating the plate thickness by a path difference of the reflected lights between the upper surface and the lower surface of the plate by using a laser beam, the measuring object obviously must be a transparent plate, so that the application of such an apparatus is limited. On the other hand, there is a method whereby a laser beam is irradiated to a plate which is not transparent from each of the upper and lower surfaces and the plate thickness is measured on the basis of the reflected lights from the upper and lower surfaces. Such a method, however, has a problem such that the operations such as an adjustment of the optical axis and the like are complicated and the plate thickness cannot be easily measured.
According to the method of measuring a plate thickness by using an ultrasonic wave, a time which is required from a time point at which an acoustic wave is generated from a signal generating source and enters the plate material as a measuring object to a time point at which the acoustic wave is reflected by the bottom surface of the plate and is returned is measured, thereby calculating the plate thickness. According to such a method, however, as the plate thickness decreases, a time difference which is caused until the irradiated ultrasonic wave signal is reflected by the bottom surface and returned is reduced, so that when a thickness of thin plate is measured, it is difficult to accurately measure such a return time. To solve the above problem, there is used a method of increasing a resolution by raising a frequency of the ultrasonic wave signal which enters the plate or the like. However, any of those methods has limits and the costs of the apparatus itself are also high.